1. Field of the Invention
The present invention relates to an electro optical modulation system utilizing electro optical effect.
2. Description of the Prior Arts
Recently, modulated light signals obtained by modulating beams such as laser beams by high frequency signals have been often used as recordings for video discs or pulse code modulation audio discs etc.
The optical modulator for modulating the beam by the high frequency signal can be an optical modulator using electro optical effect (hereinafter referring to as EO type optical modulator) and an optical modulator using supersonic optical deflection (hereinafter referring to as AO type optical modulator).
The EO type optical modulator can be used in wideband range and has high power resistance however it has disadvantages of large drift and difficulty in its operation. On the other hand, the AO type optical modulator has small drift and is easily operated, however it has disadvantages of narrowband range and inferior power resistance.
In general, it is indispensable to be wideband range for recording data in superdensity for a video disc, etc. Thus, the EO type optical modulators have been mainly used though the drift is large and the complicated operation is required.
In the conventional electro optical modulation system using the EO type optical modulator, a differential signal between a reference signal for the normal bias level and a comparison signal for a modulated condition is formed by a differential amplifier and is fed through a bias amplifier to the optical modulator, thereby controlling drift of a bias point caused by variation of the temperature, etc.
Thus, in the conventional system, it is necessary to equip splitters and optical detectors respectively at the inlet and outlet of the optical modulator, and the cost of the system is remarkably expensive and moreover, arrangement of optical axis of the optical modulator and axes of these parts is complicated. These are seriously disadvantages.